Re: [PATCH v1 03/11] perf: Allow for multiple ring buffers per event

Alexander Shishkin
2014-05-08T04:06:01Z

Peter Zijlstra <peterz@infradead.org> writes:

> How about something like this for the itrace thing?

It's much nicer than the page swizzling draft I was about to send you.

> You would mmap() the regular buffer; when write ->aux_{offset,size} in
> the control page. After which you can do a second mmap() with the .pgoff
> matching the aux_offset you gave and .length matching the aux_size you
> gave.

Why do we need aux_{offset,size} at all, then? Userspace should know how
they mmap()ed it.

> This way the mmap() content still looks like a single linear file (could
> be sparse if you leave a hole, although we could require the aux_offset
> to match the end of the data section).
>
> And there is still the single event->rb, not more.

Fair enough.

> Then, when data inside that aux data store changes they should inject an
> PERF_RECORD_AUX to indicate this did happen, which ties it back into the
> normal event flow.
>
> With this there should be no difficult page table tricks or anything.

True.

> The patch is way incomplete but should sketch enough of the idea..

Can I take it over?

> So the aux_head/tail values should also be in the file space and not
> start at 0 again, similar for the offsets in the AUX record.

With PERF_RECORD_AUX carrying offset and size, we shouldn't need
aux_{head,tail} either, don't you think?

>
> ---
> include/uapi/linux/perf_event.h | 19 +++++++++++++++
> kernel/events/core.c | 51 +++++++++++++++++++++++++++++++++++++----
> kernel/events/internal.h | 6 +++++
> kernel/events/ring_buffer.c | 8 +------
> 4 files changed, 72 insertions(+), 12 deletions(-)
>
> diff --git a/include/uapi/linux/perf_event.h b/include/uapi/linux/perf_event.h
> index 853bc1ccb395..adef7c0f1e7c 100644
> --- a/include/uapi/linux/perf_event.h
> +++ b/include/uapi/linux/perf_event.h
> @@ -491,6 +491,13 @@ struct perf_event_mmap_page {
> */
> __u64 data_head; /* head in the data section */
> __u64 data_tail; /* user-space written tail */
> + __u64 data_offset;
> + __u64 data_size;
> +
> + __u64 aux_head;
> + __u64 aux_tail;
> + __u64 aux_offset;
> + __u64 aux_size;
> };
>
> #define PERF_RECORD_MISC_CPUMODE_MASK (7 << 0)
> @@ -705,6 +712,18 @@ enum perf_event_type {
> */
> PERF_RECORD_MMAP2 = 10,
>
> + /*
> + * Records that new data landed in the AUX buffer part.
> + *
> + * struct {
> + * struct perf_event_header header;
> + *
> + * u64 aux_offset;
> + * u64 aux_size;
> + * };
> + */
> + PERF_RECORD_AUX = 11,
> +
> PERF_RECORD_MAX, /* non-ABI */
> };
>
> diff --git a/kernel/events/core.c b/kernel/events/core.c
> index 5129b1201050..993995a23b73 100644
> --- a/kernel/events/core.c
> +++ b/kernel/events/core.c
> @@ -4016,7 +4016,7 @@ static void perf_mmap_close(struct vm_area_struct *vma)
>
> static const struct vm_operations_struct perf_mmap_vmops = {
> .open = perf_mmap_open,
> - .close = perf_mmap_close,
> + .close = perf_mmap_close, /* non mergable */
> .fault = perf_mmap_fault,
> .page_mkwrite = perf_mmap_fault,
> };
> @@ -4030,6 +4030,7 @@ static int perf_mmap(struct file *file, struct vm_area_struct *vma)
> struct ring_buffer *rb;
> unsigned long vma_size;
> unsigned long nr_pages;
> + unsigned long pgoff;
> long user_extra, extra;
> int ret = 0, flags = 0;
>
> @@ -4045,7 +4046,50 @@ static int perf_mmap(struct file *file, struct vm_area_struct *vma)
> return -EINVAL;
>
> vma_size = vma->vm_end - vma->vm_start;
> - nr_pages = (vma_size / PAGE_SIZE) - 1;
> +
> + if (vma->vm_pgoff == 0) {
> + nr_pages = (vma_size / PAGE_SIZE) - 1;
> + } else {
> + if (!event->rb)
> + return -EINVAL;
> +
> + nr_pages = vma_size / PAGE_SIZE;
> +
> + mutex_lock(&event->mmap_mutex);
> + ret = -EINVAL;
> + if (!event->rb)
> + goto err_aux_unlock;
> +
> + if (!atomic_inc_not_zero(&event->rb->mmap_count))
> + goto err_aux_unlock;
> +
> + if (userpg->aux_offset < userpg->data_offset + userpg->data_size)
> + goto err_aux_unlock;

The data_{offset,size} seem to be only set by userspace too, maybe we
can do away with these altogether unless we want to allow for it to be a
sparse file?

> + pgoff = userpg->aux_offset;

..and simply do a

pgoff = event->rb->nr_pages + 1;

?

> + if (pgoff & ~PAGE_MASK)
> + goto err_aux_unlock;
> +
> + pgoff >>= PAGE_SHIFT;
> + if (pgoff != vma->vm_pgoff)
> + goto err_aux_unlock;
> +
> + /* XXX do we want to allow !power_of_2 sizes, for AUX? */
> + if (nr_pages == 0 || !is_power_of_2(nr_pages))
> + goto err_aux_unlock;
> +
> + if (vma_size != PAGE_SIZE * nr_pages)
> + goto err_aux_unlock;
> +
> + if (userpg->aux_size != vma_size)
> + goto err_aux_unlock;
> +
> + ret = rb_alloc_aux(event->rb, userpg->aux_offset >> PAGE_SHIFT, nr_pages);
> +
> +err_aux_unlock:
> + mutex_unlock(&event->mmap_mutex);
> + return ret;
> + }
>
> /*
> * If we have rb pages ensure they're a power-of-two number, so we
> @@ -4057,9 +4101,6 @@ static int perf_mmap(struct file *file, struct vm_area_struct *vma)
> if (vma_size != PAGE_SIZE * (1 + nr_pages))
> return -EINVAL;
>
> - if (vma->vm_pgoff != 0)
> - return -EINVAL;
> -
> WARN_ON_ONCE(event->ctx->parent_ctx);
> again:
> mutex_lock(&event->mmap_mutex);
> diff --git a/kernel/events/internal.h b/kernel/events/internal.h
> index 569b218782ad..6258aaa36097 100644
> --- a/kernel/events/internal.h
> +++ b/kernel/events/internal.h
> @@ -36,6 +36,7 @@ struct ring_buffer {
> struct user_struct *mmap_user;
>
> struct perf_event_mmap_page *user_page;
> + struct radix_tree_root page_tree;
> void *data_pages[0];
> };
>
> diff --git a/kernel/events/ring_buffer.c b/kernel/events/ring_buffer.c
> index 146a5792b1d2..b82505325df0 100644
> --- a/kernel/events/ring_buffer.c
> +++ b/kernel/events/ring_buffer.c
> @@ -251,13 +251,7 @@ ring_buffer_init(struct ring_buffer *rb, long watermark, int flags)
> struct page *
> perf_mmap_to_page(struct ring_buffer *rb, unsigned long pgoff)
> {
> - if (pgoff > rb->nr_pages)
> - return NULL;
> -
> - if (pgoff == 0)
> - return virt_to_page(rb->user_page);
> -
> - return virt_to_page(rb->data_pages[pgoff - 1]);
> + return radix_tree_lookup(&rb->page_tree, pgoff);

This can instead call into the underlying driver, which will likely
maintain an array similar to data_pages[] anyway.

Regards,
--
Alex